A commercially significant application of electrolysis is the production of halogens, particularly chlorine, and alkali metal hydroxides, particularly sodium hydroxide, by electrolysis of aqueous alkali metal halide solutions, particularly sodium chloride solutions, in electrolytic cells with diaphragms between the anode and cathode elements of such cells. Most cell designs contain three basic elements, namely, the anode, cathode and diaphragm. The vehicle for both supporting the anodes within the cell compartment and conducting the electrolyzing current to the anode posts is the anode base. In some instances, the anodes may be supported from the top or sides of the cell rather than be extended from the bottom, and in such cases, the top or side becomes the "base " for the purposes described herein. Commonly, the anodes are arranged vertically in uniformly spaced rows covering the width of the anode base. The cathode generally rests upon the anode base but is electrically insulated therefrom, and the cathode elements serve to divide the cell into a series of anolyte and catholyte compartments. The cathode elements additionally serve to support the diaphragm, which can be a layer of asbestos fibers or asbestos fibers and additional materials, such diaphragm serving to separate the anolyte and catholyte compartments of the cell. In the conventional diaphragm cell a relatively wide gap is maintained between the anode and the cathode, with the diaphragm separating the two. Modern cells employ a dimensionally stable foraminous metal anode, and the cathode is usually foraminous iron or steel. Each commercial cell contains a plurality of anode and cathode elements. The anode elements are fixed to the anode base and are spaced apart in a manner which permits cathode elements to be alternated between anode elements.
In the normal construction of such cells, the diaphragm is in direct contact with the cathode elements. Frequently, the diaphragm is deposited on the cathode by immersing the cathode in an aqueous slurry of asbestos, which may contain additional materials, and causing the slurry to flow through pores or mesh openings of the foraminous cathode until a porous sheet or layer of asbestos has been deposited on the sides of the cathode which in use are opposite the anode.
When the diaphragm cell is used to produce chlorine, an akali metal chloride solution is used as the cell electrolyte. As current passes through the electrolyte between the anode and cathode, chlorine is evolved at the anode and alkali metal hydroxide is formed with liberation of hydrogen at the cathode. The porous diaphragm described above, located in the anode-cathode gap, prevents mixing of the hydrogen with chlorine and mixing of formed caustic with incoming brine and chlorine.
In diaphragm cells, it is desired to keep the gap between anode and cathode small to minimize the resistance of the electrolyte in the gap to passage of electrolyzing current, and thereby reduce significantly the operating voltage of the cells and reduce energy consumption and increase power efficiency.
With conventional diaphragm cells, the gap between anode and cathode cannot be reduced below a certain practical minimum distance. Normally, a relatively wide separation between electrode elements must be specified in cell construction to allow for dimensional deviations or misalignments of the anodes and diaphragm-covered cathodes. Otherwise, such deviations or misalignments would cause scraping and injury to the diaphragm between the anode and cathode during assembly of the cell. Such scraping must be avoided to prevent breaking the diaphragm, which would cause operating problems due to mixing of the anodic and cathodic electrolysis products or electrical short-circuiting between anodes and cathodes. Cathodes, which are generally steel screens, and anodes, which are generally coated titanium mesh, become misshapen and distorted through use and with age. Furthermore, the diaphragm material is normally deposited by vacuum upon the surface of the cathode from a slurry, and the result is often a diaphragm of non-uniform thickness. Normal anode and cathode heights are more than twelve inches, and often as much as thirty or more inches, making net displacements from the vertical approaching 0.5 inch at one or more mating sites a common result as a practical matter. Therefore, when an attempt is made to place the cathode fingers, carrying the diaphragm thereon over vertically-disposed anode blades, difficulties will be encountered unless such cell are specified at a designed anode-cahode gap of approximately 0.5 inch. Difficulties include destruction of diaphragms, and the like, as pointed out in U.S. Pat. No. 3,674,676 to Fogelman.
A consideration which limits the narrowness of the inter-electrode gap relates to the passage of chlorine evolved at the anode and the replenishing of brine solution in contact with the anode. If the inter-electrode gap is too narrow in operation of the cell, cell performance can be affected adversely because of insufficient brine circulation, the formation of gas pockets, or development of hot spots. These problems are observed in operation of cells with inter-electrode gaps less than about 5 millimeters.